The present invention relates to the acquisition of data for analysis of an electrical signal, and more particularly to an improved digital trigger for capturing data samples from an input signal for analysis and display.
Traditionally test and measurement instruments that receive an electrical signal for analysis have used some type of triggering circuit to either start a display sweep in the case of analog instruments, or to initiate acquisition of a specified amount of digitized data samples from the electrical signal for analysis and display in the case of digital instruments. One specific example is a digital oscilloscope that constantly receives and digitizes the electrical signal, but only acquires the digitized data samples that exist surrounding a trigger event. The trigger event has traditionally been obtained from the electrical signal prior to digitization, i.e., from the analog electrical signal. The trigger criteria may be simple or very complex. In the digital instrument the electrical signal is digitized and stored in an acquisition memory in a recirculating fashion. When the trigger event occurs a predetermined number of samples of the electrical signal are stored in the acquisition memory after the trigger event and then acquisition stops. The contents of the acquisition memory before and after the trigger event are then analyzed and displayed.
U.S. Pat. No. 4,888,588 entitled “Digital Trigger” shows a digital trigger for a digitizing instrument having multiple analog-to-digital converters that are used in an interleaved fashion to provide multiple data samples each sample clock cycle separated in time by a specified percentage of the sample clock phase. The digital trigger includes a digital comparator for comparing the output signals from each analog-to-digital converter to a trigger threshold level, and the outputs from the comparators are then input to decoding logic which generates a digital trigger that indicates the clock phase in which the trigger point occurred. However such a trigger circuit does not provide the ability to differentiate between certain trigger events, i.e., using a single threshold level does not allow differentiation between rising edge triggers and “runt” triggers where the signal dips below the threshold and then re-crosses the threshold. Also the precision of trigger positioning is limited to the clock phase within which the trigger event occurred.
U.S. Pat. No. 5,446,650 entitled “Logic Signal Extraction” shows a technique for producing logic signal displays on a digital storage oscilloscope. An input digital logic signal is sampled to produce multi-bit digital samples. The multi-bit digital samples are processed using interpolative techniques to ascertain when the input logic signal crossed a hypothetical logic level threshold or pair of thresholds and when the logic signal was in one logic state or the other. The resulting transition times and logic states are then used as the basis for generating a variety of digital displays, including logic timing diagrams, state table displays and cursor readouts similar to those of a logic analyzer, but with enhanced resolution. However this technique is not used for generating trigger signals with precise location siting.
What is desired is an improved digital trigger that provides precision trigger positioning while having the ability to differentiate between different types of trigger events.